In most diesel engines, fuel injectors are positioned such that at least a portion of the injector tip protrudes into the engine combustion space. The injector tip is thus exposed to the high temperatures and pressures from fuel combustion and engine compression release braking. In these injectors which employ a needle valve to control the fuel spray, the valve seat can potentially be heated to close to its tempering temperature during engine compression release braking. During normal engine operation, the fuel travelling through the injector tip carries heat away. During engine braking, however, fuel spray is halted and the injector tip is thus more susceptible to heat transfer from the air in the cylinder.
Depending on the capabilities of the individual system, engine braking can be executed in a four cycle or two-cycle fashion, placing a retarding torque on the engine by forcing the pistons to compress air without a subsequent power stroke. In addition, it might be desirable to operate the engine such that the engine brake is used in combination with an exhaust valve or variable geometry turbo. In four-cycle engine braking, air is compressed within a cylinder by every other upward piston stroke. In two-cycle engine braking, air is compressed during every upward stroke. Once compressed, the air is released through an exhaust line or, in boosted engine braking, released into another cylinder via the exhaust manifold to add to that cylinder's initial mass and pressure before its compression stroke.
Boosted engine braking is a useful means of applying even higher retarding torques to the engine. However, this boosted compression of the air tends to heat the injector tip substantially, particularly in two-cycle boosting applications. In addition, the injector tip can be heated substantially during periods of simultaneous engine braking and exhaust braking. If the compressed air is allowed to heat the injector nozzle valve seat to its tempering temperature, the hardness of the valve seat material can be reduced. Because the valve seat is subjected to repeated impacts by the needle valve member, softening of the valve seat material can result in quicker wear and distortion of the seat, leading to improper sealing. Additionally, weakening of the metal in the area of the valve seat can accelerate fatigue, which can eventually lead to tip breakage and catastrophic engine failure. Exotic metal alloys with higher tempering temperatures could be used in the injector tip, however, the use of these materials is often cost-prohibitive. It is thus desirable to develop a new method of protecting the injector tip from overheating.
Heat insulating coatings and structures are known in the art and have been employed in internal combustion engines for some time. Coating the combustion chamber surfaces with a non-metallic insulator allegedly results in higher combustion temperatures and consequently more complete fuel burning. Similar coatings have been used in engine exhaust systems to maintain higher exhaust temperatures, reducing undesirable emissions. These methods appear to serve their intended purpose, which is to enhance the thermal efficiency of internal combustion engines. However, such methods are directed to treatment of relatively large surfaces within the combustion chamber, and to ensuring higher combustion temperatures rather than protecting engine components from overheating. One example of such a coating method can be found in U.S. Pat. No. 5,384,200, issued to Giles et al. on Jan. 24, 1995. The Giles method involves depositing a porous ceramic material comprised of 10%–15% volume porosity Yttria partially stabilized zirconia, or 10%–15% volume porosity Ceria-Yttria partially stabilized zirconia on a metallic layer to maintain the combustion space at a higher temperature during combustion. However, Giles does not contemplate thermal coating of the injector tip, presumably because doing to would have only a negligible effect on enhancing thermal efficiency.
The present invention is directed to overcoming one or more of the problems set forth above.